Generally, in electrophotographic image forming apparatuses, the volume of toner for use in forming toner images on a recording paper during color printing is greater than the volume of toner during monochrome printing. Therefore, in color printing, for realization of an excellent fixing of toner images on the recording paper, there is a demand for a fixing device for use in a color image forming apparatus, enhancing a property of releasing a toner from a fixing roller and a property of peeling a recording paper, or a property of melting color toners.
For example, Japanese Patent No. 3084692 (registered on Jul. 7, 2000) discloses a belt nip fixing device including a fixing roll which provides a nip portion by coming into contact with an endless belt rotatably laying in a tensioned state across the supporting rolls, the fixing device having an arrangement which enhances a property of peeling the recording paper. In the fixing device described in the publication, a pressure roll which presses the fixing roller via the endless belt is provided at an exit of the nip portion. This pressure roll, at the exit of the nip portion, causes a micro slip in an interface between a toner on the recording paper and the fixing roll, so that it is possible to peel the recording paper even in color printing, without using a peeling device.
Japanese Laid-Open Patent Application No. 42676/2001 (Tokukai 2001-42676; published on Feb. 16, 2001) and Japanese Laid-Open Patent Application No. 92296/2001 (Tokukai 2001-92296; published on Apr. 6, 2001) disclose fixing devices each having a plurality of nip portions provided when a fixing roller (heat roller) comes into contact with pressure rollers. These fixing devices have a plurality of nip portions, thereby sufficiently giving a recording material the amount of heat required for fixing, without decreasing a speed of a recording-material heating process. With this arrangement, even in color printing, toner images are melted and firmly adhered on the recording paper, thus forming fixing images in an excellent manner.
Further, in a fixing device described in the above-mentioned Japanese Laid-Open Patent Application No. 42676/2001, the rollers, which provide a plurality of nip portions, are made different in harness between each other, so that no paper jam occurs when the recording paper goes into a second nip portion downstream in the transport direction of the recording paper after the recording paper passes through a first nip portion upstream in the transport direction of the recording paper. That is, in a fixing device described in the above-mentioned Japanese Laid-Open Patent Application No. 42676/2001, when a pressure is applied to the rollers mutually different in hardness, a roller having a low hardness deforms along the shape of a roller having a high hardness. Therefore, when the recording paper passes between the rotated rollers in a state where the roller having a low hardness deforms, the recording paper is transported along the roller having a high hardness. Paying attention to this property, the above-mentioned Japanese Laid-Open Patent Application No. 42676/2001 sets degrees of hardness for the respective rollers so that the recording paper having passed through the first nip portion reliably enters the second nip portion, thus decreasing the occurrence of a paper jam.
Still further, in the fixing device described in Japanese Laid-Open Patent Application No. 92296/2001, a pressure applied to the recording paper at a nip portion upstream in the transport direction of the recording paper is lower than a pressure applied to the recording paper at a nip portion downstream in the transport direction of the recording paper. Therefore, a pressure to be applied to a recording material is lowered at an earlier stage of a heating process, and the pressure to be applied to a recording material is increased at a later stage of the heating process so that the occurrence of heat bumps in toners is suppressed, thus realizing an excellent fixing of toner images.
Yet further, a fixing device including a plurality of nip portions is also described in Japanese Examined Patent Publication No. 63795/1993 (Tokukouhei 5-63795; published on Sep. 13, 1993). The fixing device described in this publication is provided with one fixing roller and a plurality of contact rollers: primary contact roller and secondary contact roller, to perform recording on a supporting body, such as an envelope that is a combination of plural pieces of paper, without causing the supporting body to become crinkled. First, using the secondary contact roller, the supporting body is winded around the fixing roller. Then, by controlling a rotation speed of the secondary contact roller, the supporting body is transported between the fixing roller and the primary contact roller. Thus, provision of the secondary contact roller enhances a property of bringing the supporting body into intimate contact with the fixing roller, and enhances a property of fixing toner images.
In recent years, there is a demand for an image forming apparatus with reduced power requirements and reduced size. With this, there is a demand for a fixing device with reduced power requirements and reduced size, included in an image forming apparatus. Therefore, it is desired that the fixing device achieves reduced power requirements and reduced size while ensuring an excellent property of fixing toner images onto the recording paper. Especially, in recent years, a full-color printing using an image forming apparatus including a fixing device has been increasingly performed, and there is also a demand for an image forming apparatus capable of monochrome printing and full-color printing, with reduced power requirements and reduced size. To respond to such a demand, the conventional fixing device still has a further room for improvement.
For example, conventionally, in order to realize reduced power requirements of a fixing device, the fixing roller is made thin so that a heat capacity of the fixing roller can be reduced. However, the thin-walled fixing roller is susceptible to deformation caused by contact of the pressure roller with the fixing roller. Therefore, in order to prevent damage to the fixing device due to deformation of the fixing roller, it is necessary to control a nip pressure when the pressure roller comes into contact with the fixing roller.
However, under the condition where the volume of toner fixed on the recording paper is large in full-color printing and the like printing, the use of the thin-walled fixing roller does not ensure a sufficient nip pressure, causing the difficulty in ensuring an excellent property of fixing toner images. Therefore, conventionally, the thin-walled fixing roller was not suitable for full-color printing.
In addition, the conventional fixing device do enhances a property of bringing the recording paper into intimate contact with the fixing roller in an area between a first nip portion and a second nip portion (between the nips), but there is no element for pushing the recording paper to the fixing roller in the area between the nips. Therefore, there is the possibility that the recording paper could be transported without intimate contact with the fixing roller in the area between the nips.
For example, in the fixing device described in the above-mentioned Japanese Examined Patent Publication No. 63795/1993 (Tokukouhei 5-63795; published on Sep. 13, 1993), when the recording paper is transported without intimate contact with the fixing roller in the area between the nips, a leading end part of the recording paper having passed through the first nip portion which is provided by the fixing roller and the secondary contact roller comes into contact with the main contact roller. This could cause a paper jam.
Therefore, the conventional fixing device having two nip portions still has a further room for improvement that brings an excellent transport of the recording paper in the area between the nips.